In a Long Term Evolution (LTE) system, pilot measurement and data demodulation is performed by using a Common Reference Signal (CRS). That is, all users use the CRS to perform channel estimation. In case of precoding processing based on the CRS, a transmitting terminal is required to notify a receiving terminal of a specific precoding matrix (also called a precoding weight) for data transmission, which requires a high pilot overhead. Moreover, in a Multi-User Multi-Input Multi-Output (MU-MIMO) system, since multiple terminals use the same CRS, pilots cannot be orthogonal and thus interference cannot be estimated.
In an Advanced Long Term Evolution (LTE-A) system, for reducing the pilot overhead and improving the accuracy of channel estimation, a pilot measurement function is separated from a data demodulation function. Two types of reference signals are defined respectively, i.e., a Demodulation Reference Signal (DMRS) and a Channel State Information Reference Signal (CSI-RS). The CSI-RS is mainly used to perform channel measurement to obtain and feed back Channel Quality Information (CQI). Therefore, a base station side can utilize the information to complete user scheduling and implement adaptive allocation of a Modulation and Coding Scheme (MCS). Precoding information is not carried in the transmission of the CSI-RS. The DMRS is mainly used to perform channel estimation on a Physical Downlink Shared Channel (PDSCH) and an enhanced Physical Downlink Control Channel (ePDCCH). Therefore, complete demodulation of a data/control channel can be achieved. The precoding information of the corresponding PDSCH/ePDCCH is carried in the transmission of the DMRS. The LTE system and the LTE-A system can be divided into a Frequency Division Duplex (FDD) system and a Time Division Duplex (TDD) system depending on the difference between an uplink duplex mode and a downlink duplex mode. The pattern of the CSI-RS and the DMRS of the FDD system may be different from that of the TDD system.
In the LTE system and the LTE-A system, a radio frame includes frame structures in an FDD mode and a TDD mode. Link adaptation adopts a method of combining Inner Loop Link Adaptation (ILLA) and Outer Loop Link Adaptation (OLLA). The ILLA first takes charge of selecting an appropriate MCS for a User Equipment (UE). The selection is based on a mapping relationship between a measured Signal to Interference plus Noise Ratio (SINR) and the most appropriate scheme allocated. For a variety of reasons, the ILLA is not always well adapted to the channel (e.g. fast channel change), and thus a function of OLLA is necessary. The OLLA aims to achieve a target Block Error Rate (BLER) by adjusting the MCS, for example, the BLER is equal to 0.1 in the LTE system. The base station can determine the current BLER by performing statistics on a Hybrid Automatic Repeat Quest Acknowledgement (HARQ-ACK) fed back by the UE. Therefore, the method is based on HARQ-ACK feedback information of the first transmission of an HARQ.
In the LTE system and the LTE-A system, control signaling to be transmitted on an uplink has Acknowledgement/Negative Acknowledgement (ACK/NACK) and three forms for reflecting downlink Channel State Information (CSI). The three forms are a CQI, a Precoding Matrix Indicator (PMI) and Rank Indicator (RI). In the version Rel-11 of the LTE-A system, the base station can configure multiple CSI processes for the UE. The UE feeds back multiple CSIs based on the configuration of each CSI process.
The CQI plays a key role in a link adaption process. The CQI is a message sent to the eNodeB by the UE, and is used for describing downlink channel quality of the current UE. The UE can measure a reference signal sent by the base station, and then obtain the CQI by calculating.
The CQI is an indicator for measuring the downlink channel quality. In the protocol 36-213, the CQI is represented by an integer from 0 to 15, representing different CQI levels. Different CQIs correspond to their own MCSs, as shown in FIG. 1. The selection of CQI level should follow the guidelines as follows.
The selected CQI level should make the BLER, under the corresponding MCS, of a PDSCH transmission block corresponding to the CQI not exceed 0.1.
Each serial number of the CQI corresponds to a modulation mode and a transmission block size. The corresponding relationship between the transmission block size and NPRB is shown in Table 1. The CQI in the table is 4 bits. A code rate can be calculated according to the transmission block size and NPRB size.
TABLE 1CQI indexmodulationcode rate × 1024efficiency0out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
The Massive MIMO technology is a key enhancement technology in the next-generation communication technology. The main characteristics of a Massive MIMO system are as follows. A Massive antenna array is configured at the base station side, in which more than 8 antennas may be adopted to perform transmission. For example, 16 antennas, 32 antennas, 64 antennas, or even more antennas may be used to perform transmission. The transmission in case of more than 8 antennas is the Massive MIMO technology. Using such a massive multi-antenna technology is beneficial to reducing the interference between user channels. On the other hand, a large array can also bring an appreciable array gain and diversity gain. If multiple antennas are arranged on a vertical plane to form a planar array to perform transmission, the space of base-station antenna can be utilized effectively to place multiple antennas in a small space. Thereby, a horizontal gain of Massive MIMO as well as a vertical gain of Massive MIMO can be obtained. This is the 3 Dimensional Massive MIMO (3D Massive MIMO) technology or the Full Dimensional MIMO (FD-MIMO) technology.
In the current TDD LTE/LTE-A system, when the UE is configured to be a downlink transmission mode 8, a downlink transmission mode 9 or a downlink transmission mode 10, the CSI process is configured to not need the feedback of PMI/RI, and the UE only needs to feed back the CQI to the base station. The CQI is obtained based on the assumption that the base station adopts the downlink transmission mode of transmission diversity. Therefore, the UE can only feed back the CQI of a codeword stream. However, the downlink transmission actually adopted by the base station may be spatial multiplexing of multiple codeword streams, so there is a problem of non-matching between the CQI fed back by the UE and the actual downlink transmission mode. In the related art, in order to solve this problem, the base station estimates a gain of downlink multi-stream spatial multiplexing compared to the transmission diversity, and then adjusts the CQI fed back. However, in the FD-MIMO technology, with the increase of the number of transmitting antennas, it becomes more and more difficult to solve the problem of non-matching of CQIs in the related art.
Aiming at the problem of non-matching between the CQIs reported by the TDD system under the FD-MIMO technology and the CQIs actually needed in the related art, an effective solution has not been presented yet.